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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Wed, 21 Nov 2012 12:04:25 -1000
Message-ID: <CA+55aFwCiA=4+piuvf6uTT6dqeJm_Nmib_zZ=4Xj0_JmN1GrnA@mail.gmail.com>
Subject: Re: [PATCH 00/27] Latest numa/core release, v16
To: Ingo Molnar <mingo@kernel.org>
Cc: David Rientjes <rientjes@google.com>, Mel Gorman <mgorman@suse.de>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-mm <linux-mm@kvack.org>, Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Paul Turner <pjt@google.com>,
        Lee Schermerhorn <Lee.Schermerhorn@hp.com>,
        Christoph Lameter <cl@linux.com>, Rik van Riel <riel@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Johannes Weiner <hannes@cmpxchg.org>, Hugh Dickins <hughd@google.com>
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On Wed, Nov 21, 2012 at 7:10 AM, Ingo Molnar <mingo@kernel.org> wrote:
>
> Because scalability slowdowns are often non-linear.

Only if you hold locks or have other non-cpu-private activity.

Which the vsyscall code really shouldn't have.

That said, it might be worth removing the "prefetchw(&mm->mmap_sem)"
from the VM fault path. Partly because software prefetches have never
ever worked on any reasonable hardware, and partly because it could
seriously screw up things like the vsyscall stuff.

I think we only turn prefetchw into an actual prefetch instruction on
3DNOW hardware. Which is the *old* AMD chips. I don't think even the
Athlon does that.

Anyway, it might be interesting to see a instruction-level annotated
profile of do_page_fault() or whatever

> So with CONFIG_NUMA_BALANCING=y we are taking a higher page
> fault rate, in exchange for a speedup.

The thing is, so is autonuma.

And autonuma doesn't show any of these problems. Autonuma didn't need
vsyscall hacks, autonuma didn't need TLB flushing optimizations,
autonuma just *worked*, and in fact got big speedups when Mel did the
exact same loads on that same machine, presumably with all the same
issues..

Why are you ignoring that fact?

                  Linus
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